docs(docgen): add IIS default document note for Windows deployment

IIS does not serve index.html by default; omitting this config
causes the DocGen base URL to fail silently.

Author: samuelreichert

content/en/docs/marketplace/platform-supported-content/modules/document-generation/installation-configuration.md

@@ -174,3 +174,5 @@ Rule | Name | Pattern | Rewrite URL
 2 | docgen | `^(docgen/)(.*)` | `http://localhost:8080/{R:1}{R:2}`
 
 {{% alert color="info" %}}Rule 1 is based on the default URL prefix (`p`) for page/microflow URLs. If you configured a different prefix in the runtime settings of your app, adjust the rule accordingly.{{% /alert %}}
+
+{{% alert color="info" %}}If you are using IIS as your reverse proxy, make sure that the [default document](https://learn.microsoft.com/en-us/iis/configuration/system.webserver/defaultdocument/) is configured to serve `index.html`. Without this setting, the base URL will not resolve correctly and document generation will fail.{{% /alert %}}